a) Field of the Invention
The present invention relates to a method of determining an exposure time in an image forming apparatus and particularly to the same method for, utilizing fuzzy theory, determining an exposure time in the image forming apparatus such as a picture printing apparatus, for example, an apparatus for automatically printing a color film image onto color paper, a camera, a laser printer, a copying machine, a printing scanner, an electronic still camera, a video camera or the like.
b) Description of the Related Art
Conventionally, in a color picture printing apparatus, an original image recorded on a film has been divided into a multiplicity of portions for photometric purposes and, from the resulting photometric data, some image features in a predetermined plurality of areas, that is, mean density, maximum and minimum densities or the like are evaluated for a plurality of predetermined regions to determine a printing exposure based on the image feature value for each area. In this case, since it is highly probable that the main portion of an object lies in the central portion of the original image, the original image is often divided into areas including the central portion and other areas to evaluate the image feature values. Then a weighting of the image feature value in the region including the central portion is increased to determine the exposure for printing. In the case of similar original images, as shown in FIGS. 2A and 2B, although they should suitably undergo the same exposure for printing, the image features of the regions 12 including the central portion of each original image can differ. However, according to a conventional method of determining the exposure, since the exposure is determined by increasing the weighting of the image feature value in the region including the central portion, the same exposures will not be obtained for them and the exposure varies for each of the similar images. Also in photographing with a camera, in which the exposure is determined based on photometric values from a plurality of photometric elements, the same problem occurs when the position of the camera relative to the same object differs slightly, and the same is also the case with a picture printing apparatus in which when the same original images are printed a plurality of times wherein the film setting position relative to the film carrier deviates each time.
In addition, different exposure conditions or exposure operational formulas expressed in the form of a linear function of the image feature values are each defined for a plurality of previously classified patterns (for example, for the open field scene and other scenes or a picture copy and printed copy) to select one of them so that, based on the selected one, the exposure is determined. The exposure condition or exposure operational formula is selected by judging a specific pattern through a combination of the results of a plurality of judging conditions, against which the image feature value and a predetermined value are compared.
However, a slight difference in the image feature value, which is caused by a slight difference in the image variation of the measured value due to electrical, optical and mechanical fluctuations during the photometric operation, exercises a great effect on the final pattern judgment of the image. If any one of the results of the judgments differs, then the judged pattern differs with the result that the selected exposure condition or operational formula will differ to cause a fluctuation of the exposure. Further, if, in order to gain a high performance image, the pattern is classified in more detail, then the fluctuation of the exposure can further be increased while at the same time the generality of the judging conditions is degraded.
Meanwhile, in addition to the above-described exposure condition or operational formula according to the above-described pattern, a plurality of different exposure conditions or operational formulas are each defined depending on the amount of photometric data for a plurality of predefined color regions (for example, color regions on the color coordinate divided into plural numbers) or the feature value in a specific color region such as the mean density or the like. The exposure operational formula is selected according to the combination of the results of judgment, which compare the image features with predetermined values. The exposure is determined based on the selected exposure conditions or operational formulae.
However, the amount of the photometric data for the color region or the density of the same region can be changed due to only a slight difference in the images or the variation of the measured values caused by the electrical, optical and mechanical fluctuations during the photometric operation or the like, and a slight difference in the image feature values exerts a great effect on the final judgment of the color region. If any one of the results of judgment differs, then the judged color region differs with the result that the selected exposure condition or operational formula will differ to cause the color change. In addition, if there is any variation in the background color or any effect caused by the light sources of different colors, then the amount of the photometric data or the fluctuation of the density for a specific color region is reduced and a slight difference in the results of judgment can cause a major color change resulting in color degradation.
Meanwhile, in the case of the electronic still camera or video camera or the like, the camera must correspond to the light quality of illuminating the object, which is prevailing at the place where a picture is taken. Therefore, in order to reproduce a white object in white under any illuminating condition to achieve a precise color tone, there is available a white balance adjusting unit. White balance adjusting is based on the principle that when the color components over the entire image are mixed an achromatic color results, and two systems are available therefor: a system for adjusting so that an integrated value of color difference signals (R-Y) and (B-Y) may become zero and a system for detecting and controlling a white portion by comparing a luminance signal Y, and color difference signals (R-Y), (B-Y) and (Y-B-R) contained within a video signal.
However, in accordance with the above-described systems, when a single color, prevails over the image, a color failure can occur. When there is no color close to white, an erroneous control can result such as turning the color yellow or the like, which is high in luminance, into white. In addition, conventionally, an automatic picture printing machine has been widely used, in which the film image is automatically stopped relative to the printing position and is divided into a multiple areas to measure light and, by analyzing the resulting photometric data, the exposure suitable for printing is automatically determined. However, since the acceptability factor of the prints by this machine is only 70 to 98%, it is necessary to change the exposure and reprint the unacceptable prints. This reprinting is frequently needed, in particular, in a printing machine, which requires a high quality such as for an enlarged print or the like.
Although, during reprinting, the previous printing condition is modified, in many cases, an expected printing quality cannot be achieved mainly because since setting positions of the film image differ slightly between the first and second time printings, the resulting photometric data or image feature values differ which hinders the reproducibility of determining the exposure. In particular, since the film image is automatically set to the printing position at the first printing and is manually set by operator by selecting the film frame at the second printing, a deviation between the film setting positions tends to become large. This problem also occurs when the image is additionally printed (reordered).
Incidentally, it is known how to determine the exposure of camera using a fuzzy logic, but it is not clear how to set a membership function and further a highly precise and complicated reasoning has been difficult to achieve because it is expressed in terms of a pure language value having no determined value stored within the judging condition.